Due to a serious family illness I may need to get back to UK sooner than I expected.

I am on a  retirement visa which has 4 months to run, the nature of the illness means I have to stay there until "The End!" so my stay is going to be undetermined.

Question is can I change from retirement to marriage visa whilst the former is still current (I have all requirements for a marriage visa) thus allowing me a full years grace with only a re-entry visa to get stamped in my passport.

You could ask immigration about changing the reason for your extension of stay (it is not a visa) from retirement to one based upon marriage to a Thai early.

Normally they would not do it until your extension is close to the day it expires.

If they do not do it you could apply for a non-o visa based upon marriage at the embassy in London. And then apply for a new extension of stay.